In a communications system, it is often necessary to establish frame synchronization with the received signal. Frame synchronization is the correct temporal alignment of a data detector with the formatted, transmitted data. Typically, a known signal pattern, referred to as the synchronization mark, is transmitted on a communications channel. The receiver contains a circuit, referred to as a synchronization mark detector, that detects a synchronization condition when the synchronization mark is recognized. Thus, the synchronization condition can be used to synchronize the receiver with the transmitted data following the synchronization mark in the received signal. In a magnetic recording system, for example, data sectors on a magnetic disk are formatted to include an acquisition preamble, followed by a synchronization mark and then user data.
The synchronization mark detectors used in conventional magnetic recording systems employ a sequence detector, such as a Viterbi detector, to estimate the written binary data (e.g., NRZ data). The sequence detector simply counts the number of bit differences between a block, b−L+1 . . . b−1b0, of NRZ bits comprising the synchronization mark, and each block, {circumflex over (b)}i−L+1 . . . {circumflex over (b)}i−1{circumflex over (b)}i, of bits estimated by the sequence detector (where i ranges over a synchronization search window). The number of bit differences between the expected sequence and the estimated NRZ sequence, referred to as their “Hamming distance,” is compared to a preset threshold. The synchronization condition is asserted when the Hamming distance falls below this threshold.
The Hamming distance detector has two significant drawbacks. First, if the Viterbi detector needs to be calibrated, e.g., in the case of a noise predictive Viterbi detector, then the detector performance depends strongly on the quality of the Viterbi calibration. Moreover, the known data mode of calibration is next to impossible without prior frame synchronization. Second, the Hamming distance detector has poor performance in DC offsets when the equalization target has DC content, as is the case for perpendicular magnetic recording systems.
A need therefore exists for improved methods and apparatus for performing synchronization mark detection. In particular, a need exists for synchronization mark detectors that do not depend on a NRZ sequence detector. A further need exists for synchronization mark detectors that are robust against DC offsets even when the equalization target has DC content.